The idea that veteran gay rights activist Peter Tatchell has been pulling David Cameron’s strings over the gay marriage bill, inspiring the Prime Minister to action and even writing supportive lines for his speeches, seems absurd. Yet it’s a notion that carries much weight within some Conservative circles at Westminster. In the House of Commons debate earlier this week, MP for mid-Bedfordshire Nadine Dorries even demanded that Maria Miller, the Minister for Women and Equalities, categorically deny the rumours. “Will the Minister please tell us that Mr Tatchell has not inspired this Bill, and that it is not based on his words ...” she asked crossly.

Tatchell is not a man to stand shyly aside and refuse credit where it’s due. And, yes, he was at the centre of the intense lobbying that eventually won the day on gay marriage, he claims. But it wasn’t David Cameron whose influence and support for the issue he valued most of all. It was that of London Mayor Boris Johnson.

“Boris’s backing for same sex marriage was a game-changer,” declares Tatchell simply.

It was in July 2010 that the Mayor memorably headed the annual Gay Pride march through central London, flanked by tattooed men in leather thongs and wearing an ill-advised pink cowboy hat. At the end of the march, Tatchell — ever the stunt-puller —ambushed Johnson in front of the world’s media and “asked him directly whether he would support an end to the ban on same-sex marriage”. Tatchell adds: “He stumbled for a few seconds and then said yes. And that was hugely important. With Boris Johnson supporting marriage equality, it was suddenly respectable in large sections of the Conservative Party. He’s the second most influential Tory politician after Cameron. Not all Tories love Boris, but very many do.”

A few days later, Tatchell did it again. Invited to a reception for influential Londoners at City Hall he jumped onto the stage after Johnson’s welcoming speech and thanked the Mayor for “supporting” gay marriage.

“And the entire audience burst into cheers and applause. That was really significant too.”

At that stage, the Prime Minister, the Chancellor, George Osborne, and Home Secretary, Theresa May, were all publicly opposed to legalising gay marriage. May and Osborne had met Tatchell before the election and promised a review of government policy on the issue should they win power, but three months after forming the Coalition, informed him that the review had taken place and that no change would be made.

However, bounced or not, Boris was behind it — but what made the PM, Osborne and May change their minds?

Tatchell is well-placed to reveal the behind-the-scenes manoeuvring that led to Tuesday’s historic vote (of which more later), if only because he has campaigned so hard on the issue for so long — his Equal Love campaign is more than 10 years old — and he has his spies.

His career in activism now spans more than 40 years; he famously works a seven-day week from his council flat in Bermondsey and subsists on less than £10,000 in donations from well-wishers. To social conservatives he is bogeyman-in-chief (the journalist Charles Moore calls him “an energetic crank”) but within the gay community, and to many on the Left, he is a brave and principled exponent of direct action.

The general public knows Tatchell’s long, lugubrious face, almost always seen beside a placard or banner, largely from news footage of violent scuffles. He has been assaulted many times, and horribly beaten twice — once by thuggish bodyguards while attempting a citizen’s arrest of Robert Mugabe in Brussels, and again during an ugly attempt to export Gay Pride to Moscow. He has minor brain damage as a result, and last month had an operation to restore some of the sight lost in his right eye. When he smiles, Tatchell reveals, at 61, what look like expensively veneered teeth. In fact they come courtesy of the NHS: “Over the years I’ve been bashed so many times nearly all the teeth in my mouth have been chipped and cracked. Most of the teeth you see are reconstructions.”

Yet this is very much his moment. Historically, Tatchell has barely been tolerated within senior political circles — he claims both Tony Blair and Gordon Brown expressly excluded him from the Downing Street reception held every year on the eve of Gay Pride — but there are clear signs of rehabilitation.

In a society so changed since he first chalked a slogan in the late Sixties, few of those early campaigns seem so radical today. “When I first became aware of the vast array of anti-gay laws on the statute book, I resolved to dedicate a part of my life seeking their repeal,” he says. “I calculated that it would take 50 years to win gay equality. But it turns out I was a bit of a pessimist. Once we get same sex marriage, that goal will have been achieved.”

The irony is, Tatchell would never get married himself — even if he had a long-term partner. “Personally I’m not a great fan of it,” he declares. “I feel uncomfortable with the sexist patriarchal history of marriage — originally it had nothing to do with love, it was about property and male social power over women. Just look at the language. An alternative meaning for the word ‘husband’ is to manage and control, which symbolises the way men have traditionally treated their wives.”

Which all sounds very Tatchell-esque and quite irrelevant nowadays, yet is also the logical underpinning for his continuing campaign to give straight couples the legal option of a civil partnership. It is the heteros, after all, who now face discrimination on that score.

There is a sadness, however, to the thought that he might have sacrificed his own relationships for the sake of other men’s right to marry men. There’s been little time in his life for long-term partnerships, he says. “It’s a curious contradiction. There I am fighting for the rights of gay people to be able to love and have relationships that are recognised and validated by the state, yet I am not in such a relationship myself. It might be viewed as odd, yes. For many people, having a long-term partner and children is an essential element of their personal fulfilment, and I respect and support that. But it’s just not for me.”

Back to Westminster and the apparent conversion of the PM, the Chancellor and the Home Secretary. In late 2010, continues Tatchell, he arranged for four same-sex couples and four straight couples to file applications at local registry offices for, respectively, civil marriage and civil partnership licences. All were refused, of course. In February 2011, Tatchell and human rights lawyer Robert Wintemute of King’s College London applied to the European Court of Human Rights to strike down the bans that stymied those eight couples, and told the government that, sooner or later, it would have to go to Strasbourg to defend the existing law. Meanwhile, lobbying harder than ever, Tatchell wrote a “briefing paper” extolling gay marriage as a natural extension of the Conservative Party’s traditional support for the existing institution of marriage — and, with a fair amount of chutzpah, circulated it widely within Tory circles.

“That’s when things started to happen,” he says. “Astonishingly, within three months of our application to the European Court, the Government announced that it was going to consult on legalising gay marriage. They knew that there was no argument they could use in Strasbourg that would be anything other than bigoted and intolerant. I think they realised the game was up, and decided it was better to lead on the issue and get the kudos of enacting liberal reform than be dragged through the courts. It may have been a pure coincidence, but it does strike me as very closely mirroring the pattern of events that I set in place.”

There was more mirroring — perhaps — in David Cameron’s stirring speech at the Conservative Party Conference in October that year. As the Prime Minister for the first time announced himself in favour of gay marriage, Tatchell claims that Cameron used in the speech a number of the ideas contained in that “briefing paper” sent out to Tories.

“The Prime Minister’s arguments were not exactly expressed in the words used in my document,” claims Tatchell. “But they were pretty close. [His speech] certainly echoed the briefing.” And herein lies the root of those Chinese whispers repeated by the likes of Nadine Dorries in the Commons earlier this week. True or not, they make for an interesting study in paranoia (and self-importance).

But it is not done yet, says Tatchell. There are plenty more quarrels to be had, not least with the new Archbishop of Canterbury, Justin Welby, whom Tatchell accuses of being “blatantly homophobic” for his reiterated opposition to gay marriage.

Yet there will also be plenty of weddings to go to, presumably. Tatchell is good friends with Elton John and David Furnish, and will no doubt get an invitation to theirs. We muse briefly on where they might have it — at their lovely home near Windsor, surrounded by all that art? At the pad in the South of France, if the French follow our lead?